The present invention relates to a process for the preparation of polymeric absorbents useful for the gelling of organic liquids. More particularly, the present invention relates to a process for the preparation of polymeric absorbents which are cross-linked (5 to 50%) and are capable of absorbing organic liquids in the presence of a ppm level metal additive. The polymers prepared by the process of the present invention have a proper balance of hydrophilic and hydrophobic character in its chemical structure. The polymers prepared by this technique are known in the art as polymer gels or more commonly and hereinafter referred to as gels.
Polymer gels consist of cross-linked macromolecules that form a three dimensional network in which solvent molecules are absorbed by osmotic forces. Polymer gels have an equilibrium absorption capacity, which is governed by a balance of osmotic pressure of the solvent and the elastic stress of the network. Polymer gels, which absorb water, are referred to as hydrogels. Those hydrogels, which can absorb large quantities of water, such as in excess of 100 gram of water per gram of dry gel, are called superabsorbants. Polymeric superabsorbents have been widely used in personal care products such as sanitary napkins and diapers. They have also been used in agricultural applications such as for irrigating dry and arid land. Recent developments and new applications of superabsorbents have been outlined in literature (Bucholz, F. L., Chemtech, Sep. 38, (1994)). Superabsorbing gels are prepared by polymerising highly hydrophilic monomers such as acrylamide and acrylic acid along with a small quantity of multifunctional monomers and using a suitable initiator, typically a radical initiator, in water as a solvent.
Superabsorbing hydrogels do not absorb organic solvents. In fact, many of them collapse in organic solvents such as acetone and alcohol. Gels, which can absorb organic solvents, have been developed to a limited extent. Examples of such gels are chlorosulfonated polyethylene gels (Varma, A. J., Lele, A. K. and Mashelkar, R. A., Chem. Engg. Sci.,50, 3835 (1995)) and polyethylene oxide gels (Graham, N. B., Nwachuku, N. E. and Walsh, D. J., Polymer , 23, 1345 (1982)).
There are several advantages of absorbing organic liquids into gels. For example, the liquid can be transformed into a soft solid by entrapping into a three dimensional matrix of a gel. Soft solids have typical properties that are intermediate between solids and liquids. For example, gelled organic liquids can have a high viscosity as well as a finite modulus. In general, gelled liquids find applications in pastes, lotions, creams, shampoos, oil drilling fluids and in fuels. Gelled fuels are particularly useful for cooking and chafing dishes. They are easier and safer to transport and are more effective owing to the slower diffusive release of the fuel from the gelled matrix. Such gelled fuels typically contain an alcohol, usually methanol or ethanol as the fuel, which may be mixed with other C1 to C6 alcohols. It is desirable to gel the fuel in such a manner that it does not separate from the matrix on standing or on application of pressure.
Alcohol based gels have been formed by different gelling agents. U.S. Pat. No. 3,754,877 discloses the use of olefin modified hydroxyalkyl cellulose as the gelling agent. U.S. Pat. No. 4,436,525 discloses the gelling of a 3:1 mixture of methyl alcohol and isopropyl alcohol with a fatty acid soap and sodium hydroxide. U.S. Pat. No. 3,759,674 discloses the dispersions of ethylene-acrylic acid copolymers and amine emulsifiers in water, which form gels when mixed with alcohols. These gels contain about 10 to 30% by weight of solids. U.S. Pat. No. 3,148,958, U.S. Pat. No. 4,261,700 and U.S. Pat. No. 4,365,971 disclose the use of carboxy vinyl polymers such as Carbopol ethylene acrylic acid copolymer partially neutralised by weak amines as gelling agents for alcohols. U.S. Pat. No. 5,641,890 discloses the use of an amine neutralised anionic polymer such as Carbopol 676 along with an amphoteric Theological additive such as dispersed alumina for gelling alcohol. The gel so formed contains about 20 to 30% by weight of water. The gel is formed by non-covalent physical cross-links originating from interactions between the polymer and the amine. A typical gel contains about 70 weight % alcohol and about 1 weight % each of the polymer and the amine.
While the disclosures referred to above are interesting, they do not provide any teaching as to the structural features of the polymer absorbent. Also, no teaching is provided for synthesizing a solvent absorbing covalently cross-linked polymer gel. Whereas the disclosures of the above references have used a commercial polymer, in the present invention the polymer gel is prepared in situ by polymerizing selected monomers which contain a proper balance of hydrophilic and hydrophobic character in the presence or absence of a suitable transition metal compound. The proper choice of monomers containing hydrophilic and hydrophobic functional groups is particularly necessary for absorption of C1 to C6 alcohol and their mixtures, without the need for presence of water. Also in the prior art, the gels prepared inevitably required additional gels in order to increase the viscosity thereof.
The gel of the invention does not require any other gelling agents besides the polymer itself. The gelled fuels may or may not contain any water. Absence of water can significantly improve the burning characteristics thus giving an improved gel. Polymerisation of the monomer in the presence of a suitable transition metal compound helps in controlling the level of cross-linking reaction as well as forming structural complexes with the organic liquid. This improves the absorption capacity of the polymer for the organic liquid. The gel so prepared does not require any other gelling agent or thickening agent to increase the viscosity besides the polymer unlike the disclosures of the above references.
It is an object of the invention to provide a polymer gel that can osmotically absorb large quantities of organic solvents to form gelled liquids.
It is another object of the invention to provide polymer gels that can be used for the formation of gelled fuels.
It is further object of the invention to provide a process for the preparation of polymer gels that can be used for preparing gelled fuels useful in cooking and heating food.
It is yet another object to provide polymer gels that do not require further gelling and/or thickening agents to increase the viscosity thereof
It is a further object to provide a process for the preparation of polymer gels that have improved absorption capacity for organic liquids, particularly organic solvents.
It is another object of the invention to provide a process for the preparation of polymer gels that absorb C1 to C6 alcohols or mixtures thereof without the need for the presence of water.
Accordingly, the present invention provides a process for the preparation of polymeric absorbents useful for gelling organic liquids which comprises mixing one or more monomers with a cross-linking agent, a free radical initiator, an optional solvent, optionally in the presence of a transition metal source and subjecting the mixture so obtained to a conventional polymerisation method, removing the polymer, crushing the polymer to obtain polymer powder, washing with solvent, drying the polymer by conventional methods to remove unreacted monomers, followed by swelling in alcohols to obtain the desired product.
In one embodiment of the present invention, the process is optionally carried out in the presence of a transition metal source.
In yet another embodiment of the invention, the source of the transition metal may be metal, metal salts or metal complexes of cobalt, chromium, copper, manganese and iron.
In a further embodiment of the invention, the transition metal source is selected from chromium trioxide, cobalt chloride, manganese hydroxide and ferric oxide.
In a further embodiment of the invention the amount of the transition metal used is between 5 ppm to 500 ppm, preferably between 10 ppm to 250 ppm.
In a further embodiment of the invention, the process is optionally carried out in the presence of a solvent.
In another embodiment of the invention, the solvent used for polymerization are polar or non-polar solvents selected from water or an aqueous mixture of alcohols, 1,4-dioxane, dimethyl sulfoxide and dimethyl formamide, benzene and xylene.
In another embodiment of the invention, the monomers used have a general formula CH2xe2x95x90CHxe2x80x94R1xe2x80x94R2xe2x80x94R3 wherein R1 and R3 are hydrophilic groups that may be either one or a combination of amide, ester, sulfonic acid, carboxylic acid and hydroxyl functional groups, and R2 may be a hydrophobic group chosen from one or a combination of primary, secondary or tertiary aliphatic saturated or unsaturated hydrocarbons, aromatic hydrocarbons, or cycloaliphatic hydrocarbons selected from acrylamide or their derivatives exemplified by various monomers given in Table I below:
In yet another embodiment of the invention, the amount of the comonomers in the polymerisation mixture may be varied between 1 mole % to 99 mole %, more preferably between 9 to 91 mole %.
In a further embodiment of the invention, the cross-linking agent may be acrylic/methacrylic or styrenic in nature or mixtures thereof and have two or more unsaturations such as N,N-methylene bisacrylamide, ethylene glycol dimethacrylate, ethylene glycol diacrylate, trimethylol propane triacrylate, trimethylol propane trimethylacrylate, divinyl benzene and more preferably N,N-methylene bisacrylamide and ethylene glycol dimethacrylate.
In a further embodiment of the invention, the mole, percent of the multi-functional monomer may be typically between 0.1 mol % and 40 mol %, preferably between 1 mol % and 30 mol % and most preferably between 5 mol % and 20 mol %.
In yet another embodiment of the invention, the initiator used for thermal polymerisation is selected from azo, peroxides, hydroperoxides and persulphates, preferably from azo initiators and persulphates.
In another embodiment of the invention, tetramethyl ethylenediamine in an amount between 1% to 4% of the total feed is used as the polymerisation accelerator along with persulphate initiator in water as the solvent.
In yet another embodiment, the polymerisation of the polymer gel is carried out preferentially thermally between 50 to 90xc2x0 C. and more preferably between 50 to 70xc2x0 C.
In yet another embodiment of the invention, the polymer gel may be prepared in water-alcohol mixture in the composition range of 0 to 100 volume percent of alcohol, preferably between 0 to 75 volume percent alcohol. The concentration of the monomer in solution may be varied between 5 to 50 wt. percent, preferably between 5 to 20%.
In another embodiment of the invention, the solvent used for washing is selected from water, acetone, or an aqueous mixture of alcohols.
In a further embodiment of the invention, the polymer gel of the invention is crushed to a soft mass after polymerisation.
In another embodiment of the invention, the organic solvents include aromatics, acids, ketones, alcohols, glycols and amines.
In a further embodiment of the invention, the absorbable organic liquid is selected from aliphatic alcohols, C1 to C6, preferably C1 to C3 and more preferably C1 to C2.
The preferred range of modulus of the swollen polymer gel is between 0.1 to 2 MPa and a preferred range is between 0.2 to 0.6 MPa.
The gelled solvent contains 0.1 to 10 weight percent of a polymer gel, the rest being the organic solvent and/or their mixtures that are osmotically absorbed in the gel. The polymer gel contains hydrophilic and hydrophobic functional groups and is preferably formed by polymerising a vinyl monomer having a chemical structure of the type CH2xe2x95x90CHxe2x80x94R1xe2x80x94R2xe2x80x94R3 wherein R1 and R3 are hydrophilic groups and R2 is a hydrophobic group. The polymerisation is preferably carried out in the presence of ppm levels of a transition metal compound that can complex with the polymer.
Polymerisation can be conducted in any conventional manner. For example, polymerisation can be done by thermal polymerisation, photochemical polymerisation, solution polymerisation, bulk polymerisation, suspension polymerisation, emulsion polymerisation and precipitation polymerisation. In thermal polymerisation the initiator is activated by supplying thermal energy. Photochemical polymerisation may also be done using a radiation source.
Bulk polymerisation is carried out by dissolving the initiator in the liquid monomer. In solution polymerisation, the monomer and the initiator are dissolved in a suitable solvent in which the resultant polymer is soluble or swellable. In the case of suspension polymerisation, the insoluble monomer is suspended in water in the form of droplets with the help of a suspending agent and then polymerised.
In emulsion polymerisation, the monomer is dispersed in aqueous phase as a uniform emulsion and then polymerised. Alternatively, the resulting polymer can also be precipitated from the reaction mixture.
The separation of organic liquid from the gel by the action of temperature of pressure is not preferable. In the present invention, the organic liquid is osmotically absorbed in the gel due to the action of strong intermolecular forces. The gel does not contain macroporosity and hence the solvent cannot separate from the gel on the application of pressure. The choice of monomers also ensures that the organic solvent does not separate from the gel due to the change in temperature.
The process of the present invention will now be described with reference to the following examples, all of which are illustrative and are not to be construed as limiting the scope of the invention in any manner.